The foundation of your Allan Block retaining wall is as you can imagine, critical.
When excavating you must dig down to undisturbed soil. If over excavating has to be done for site services, or other reasons soil cannot be added back to raise the grade. Instead you need to add compactable gravel, until you reach the desired elevation.
Soil preparation for a retaining wall
For a typical retaining wall, will need to excavate 10”-12” of soil. This provides enough room for a minimum of 6” of gravel, 4” of buried block.
Once excavated the ground will be flat, and clean of all organic matter. Remove any big rocks so a compactor can rest completely on the soil. Compact the area as much as it requires.
In some cases, the soil can be flat, while other times you may want a slope to aid in the drainage of the “trench”. Instal any drainage pipe directly on the soil, and at the back of the trench making sure it won’t be under the block wall.
Once the soil is clean, compacted, and the drainage pipe if required is in begin to import the angular ¾” clean gravel. If you have a small compactor import the gravel in small layers and compact.
The bigger your compactor,the deeper the gravel can be before you compact. This is called compacting in lifts. 6-8” lifts are normal for small compactor under 200lbs. a minimum of 2 passes is required.
You should use water to keep the dust down, and to lubricate the rocks as they rotate to lock together.
We call the ¾” chunky gravel the subbase, and the last layer of gravel we call the base.
The base is a thin (1”-1.5”) layer of ¼” clear gravel that is much easier, and better to perfectly level.
You can do this by hand, but using a laser level, and a screed bar setup produce PERFECT results.
With a perfectly prepared base you should be able to base each block in about 5 min, while a bigger rock sub base will take longer.
Size and depth
The size and depth will vary a lot depending on site conditions. If building on poor soil you will need a much deeper, and wider base.
Typically this is about 6” minimum of gravel, and about 30” wide. This gives you room for 12” of drainage gravel behind the wall, 12” of room for the Allan Block itself, and 6” of working room in front of the wall.
The size of this Base trench will affect the quality, lifespan, and cost of the retaining wall.
The bigger the base, the stronger your wall will be (within reason of course).